1. Field of the Invention
The present invention relates to the field of wireless communications.
2. Description of Related Art
Wireless networks typically rely on relatively short-range transmitter/receiver (xe2x80x9ctransceiverxe2x80x9d) base stations, each connected to a switching center, to serve mobile subscriber terminals in small regions (xe2x80x9ccellsxe2x80x9d) of a larger service area. By dividing a service area into small cells with limited-range transceivers, the same frequencies can be reused in different regions of the service area, and mobile terminals which consume relatively little power can be used to communicate with a serving base station. Service providers of such wireless networks incur substantial costs to establish the dense pattern of base stations needed to ensure adequate service, including the cost of buying/leasing the property on which base stations and switching centers are located, the cost of licensing the frequency bandwidth used for air-interface channels, and hardware/software costs associated with each base station, switching center, and landline connections between switching centers and base stations.
A significant percentage of the cost for a single base station is the cost of the antenna structure used to transmit/receive radio frequency (RF) signals to/from wireless subscriber terminals. The specific antenna structure used depends on various factors, such as cell radius (e.g., requiring a high-gain antenna structure), whether the cell is sectorized (e.g., a number of directional antennas may be used for a sectorized cell while an omni-directional antenna may be used for a non-sectorized cell), and whether diversity reception is implemented.
For many geographic regions, particularly metropolitan regions, consumer demand for wireless services can support several coexisting wireless systems, each allocated a different block of frequency spectrum. Such coexisting wireless systems will typically have independent network infrastructures and use separate antennas which provide mutual isolation. Because each base station must filter out frequencies which are not in their allocated transmit/receive bands and because transmit amplifier specifications set limits on acceptable spurious noise levels, for example to comply with FCC (Federal Communications Commission) regulations, communications from base stations/mobile subscriber terminals of first and second wireless systems will typically not interfere with each other when using separate antennas.
In rural regions, and for marginally competitive service providers, infrastructure costs may preclude establishing or expanding wireless network service in a given geographic area because of a limited number of subscribers. To address the substantial costs required to establish a wireless network, and thereby improve a service provider""s ability to establish/expand their network service area, it has been proposed to share antenna structures between multiple service provider base stations, recognizing that base stations of different wireless systems will transmit/receive on different RF frequencies.
Despite the filtering circuitry of individual base stations (e.g., using a duplexer arrangement having a first band pass filter which passes frequencies in the transmit band and a second band pass filter which passes frequencies in the receive band) and transmit amplifier specifications which limit acceptable spurious noise levels at frequencies outside the allocated block of spectrum, the frequency bandwidths allocated to different wireless systems may be near enough that the conventionally-implemented filtering performed by each base station will be insufficient to prevent interference between the communication signals of each wireless system in a shared antenna environment. Additionally, the physical connection of transmission lines from multiple base stations at a common connection point will generally cause considerable power loss (xe2x80x9cinsertion lossxe2x80x9d), as much as 50% loss, attributable to the transmit/receive signal of one system feeding into the transmission line of the second system. Such insertion loss will require increased power and/or a higher gain antenna structure to achieve acceptable signal-to-noise characteristics.
The present invention is a system and a method for effectively combining communications of the base stations of multiple wireless systems on the same antenna structure. In one embodiment, the present invention is a wireless system combiner which serves as an interface between base stations of first and second wireless systems (xe2x80x9cfirst base stationxe2x80x9d and xe2x80x9csecond base stationxe2x80x9d) and a shared antenna to substantially eliminate spurious noise from the first base station at frequencies allocated to the second base station and further to prevent transmit power from the first base station from feeding into the reception circuitry of the second base station in a shared antenna configuration.
The combiner according to one implementation of the present invention includes a first combiner filter connected between a duplexer of the first base station and a common connection point and a second combiner filter connected between a duplexer of the second base station and the common connection point. The first combiner filter in this implementation filters out spurious noise generated by first base station transmitter at frequencies outside the frequency band allocated to the first base station, for example using a high Q value band-pass or band-reject filter. The second combiner filter in this implementation filters out signal power at frequencies outside the second base station receive band to prevent transmit signal power of the first base station from feeding into the second base station""s receiver circuitry, thereby preventing intermodulation.
The first and second combiner filters may be implemented as discrete elements from the circuitry of each base station, thereby allowing service providers of each wireless system to design their base station, and in particular base station transmit amplifier and filtering circuitry, without regard to whether the base station will be implemented in a shared antenna environment. Alternatively, the first and second combiner filters may be incorporated in the filtering circuitry of the first and second base stations respectively.
Still further, the first and second combiner filters according to embodiments of the present invention significantly decrease insertion loss (i.e., the power loss resulting when the transmission lines for each base station are connected at a common point between the antenna structure and the individual base stations) by creating very high impedance in the first base station side of the shared antenna configuration for frequencies of the second base station, and vice versa. Insertion loss can be even further reduced by achieving an electrical length of the transmission line between the first/second combiner filter and the common connection point which is tuned to the frequencies allocated for the first/second base stations respectively. As such, transmit/receive signal power for each of the first base station and the second base station will not substantially be lost in the other base station side of the shared antenna configuration.
In one exemplary implementation, a base station of a CDMA (Code Division Multiple Access) system, e.g., operating in accordance with the IS-95 A/B CDMA standard, and a base transceiver station of a GSM (Global System for Mobile communication) system are connected to the same antenna structure via a combiner. Base stations for CDMA wireless systems are typically allocated a receive band of 825 MHz-835 MHz and a transmit band of 870 MHz-880 MHz (for xe2x80x9cA-Bandxe2x80x9d) while base stations of GSM wireless systems are typically allocated a receive band of 890 MHz-915 MHz and a transmit band of 935 MHz-960 MHz. Even after each base filters out frequencies which are not in their respective transmit and receive bands, spurious noise from the CDMA base station transmitter will exist at receive frequencies of the GSM base station (e.g., at 890 MHz) due to the performance of the CDMA base station""s transmit amplifier and the roll-off characteristics of filters typically used by a CDMA base station. Furthermore, CDMA base station transmit power in the range of 870 MHz-880 MHz will directly feed into the GSM base station receiver in a shared antenna configuration if not addressed, thereby degrading GSM receive performance. First and second combiner filters according to the present invention address these drawbacks by substantially eliminating spurious noise from the CDMA base station at frequencies allocated to the GSM base station, and preventing transmit power from the CDMA base station from feeding into the reception circuitry of the GSM base station.